Federal, state and local governmental bodies reacting to constituent pressures have instituted a series of laws and regulations aimed at preventing the continued contamination of the environment. Heavy metals are generally defined as hazardous and, therefore, must be removed from industrial effluent streams. Once removed from these streams, the heavy metals-containing waste has been containerized and then disposed of in government-sanctioned landfills. These special landfills are now being more closely monitored thereby forcing alternative methods of disposal of these solid heavy metal wastes. It is toward both the clean-up of these effluent streams and discontinued pollution of soil and ground waters that the invention of this method is directed.
The metals plating and finishing industries have been moving to a "zero-discharge" basis for disposal of metal-bearing wastewater streams. Progressively stricter regulatory criteria have forced industry to drastically reduce the residual metal contents in wastewater discharges, while the increased cost of disposal of solid metal wastes have forced industries to examine the economic benefits of both "zero-discharge" for effluent waste streams and of the recovery and recycling of the removed heavy metal from these waste streams. As a result, instead of discharging the "used" industrial process water directly into municipal sewer systems or directly into natural bodies of water, this "used" water is to be recycled through the plant or manufacturing facility that generated it for treatment and recovery.
The ability of conventional wastewater treatment methods to achieve the low levels of residual metals required by the higher standards for wastewater purity in many cases is marginal. In addition, most conventional methods of heavy metals removal produce copious amounts of sludge, usually classified a priori as a hazardous material. Recent legislation has made the disposal of sludge material extremely difficult and expensive and no near term solution to the sludge disposal problem seems apparent.
Because of these problems, industry in general, and the metal plating and finishing segments in particular, have been forced to consider alternative methods for heavy metals removal from wastewater streams. The major characteristics needed in heavy metals removal from wastewater streams are: ability to reduce residual metal contents to extremely low levels (ultimately to the parts-per-billion range); production of minimal amounts of sludge, from which metals can be reclaimed and recycled; economical operation; production of effluent suitable for recycle to process operations; and ability for maximum retrofit into existing installations.
One of the more promising new alternative approaches that possesses the potential of fulfilling to a significant degree these desirable requirements for treating metal-bearing wastewaters is xanthate technology. A patent to John Hanway Jr., et al, U.S. Pat. No. 4,166,032, discloses the use of cellulose xanthate for heavy metals removal from wastewater streams. While cellulose xanthate is very effective for the removal of heavy metals from wastewater, the cellulose xanthate adds an amount of sludge equal to the dry weight of the cellulose xanthate added to the wastewater stream further increasing both the weight and volume of the sludge generated and to be treated. In accordance with the present invention, it has been found that trithiocarbonates, and particularly alkali metal trithiocarbonates, effectively precipitate heavy metals from wastewaters leaving a substantially nonpolluted solution or effluent capable of plant recycle or legal discharge. In addition, the low volume of generated sludge can be easily recycled using existing technology.